Latched comparator circuitry

ABSTRACT

Integrated circuits having analog-to-digital converters are provided. Analog-to-digital converters may contain latched comparators. A latched comparator may include inputs configured to receive a differential input voltage signal, a differential reference voltage signal, and a clock signal. The comparator may include a preamplifier, a latching circuit, a level shifter, and a flip-flop coupled in series. The preamplifier may include large input transistors for minimizing offset, stacked tail transistors, and diode-connected load transistors for minimizing kickback noise. The preamplifier may be used to generate amplified voltage signals. The latching circuit may include a first pair of cross-coupled pull-down transistors, a second pair of cross-coupled pull-up transistors, and precharge transistors. The precharge transistors may serve to precharge the latch output to a predetermined voltage level during a first clock phase, whereas the first and second transistor pairs may serve to perform exponential regeneration on the amplified voltage signals during a second clock phase.

BACKGROUND

Integrated circuits often include data converters such asanalog-to-digital (A/D) converters. An analog-to-digital converter is adevice that is configured to convert a continuous (analog) signal to adiscrete time digital signal. For example, an analog-to-digitalconverter can be used to convert an input analog voltage (or current) toa corresponding digital representation that is proportional to themagnitude of the input voltage.

There are many different types of analog-to-digital converters. Asexamples, the different implementations include flash A/D converters,single-slope A/D converters, dual-slope A/D converters, successiveapproximation A/D converters, pipeline A/D converters, oversampled A/Dconverters, etc. A majority of these A/D converters includes some typeof comparator circuitry. Comparators are fundamental building blocks ofA/D converters. A comparator is a circuit that has a first input thatreceives a first input signal, a second input that receives a secondinput signal, an output, and power supply terminals (i.e., terminals onwhich a positive power supply voltage and a ground power supply voltageare provided). The comparator will drive its output signal high or lowdepending on whether the first input signal is greater or less than thesecond input signal. For example, if the first input signal is greaterthan the second input signal, the comparator will drive the outputsignal to the positive power supply voltage level (i.e., a logic “1”).If the first input signal is less than the second input signal, thecomparator will drive the output signal to the ground power supplyvoltage level (i.e., a logic “0”).

A comparator can therefore be defined as a high gain amplifier having adifferential analog input and a large swing output (i.e., the outputsignal should be able to swing from the ground power supply voltage allthe way up to the positive power supply voltage and vice versa). Aconventional comparator is formed using multiple amplifiers cascaded ina chain (i.e., multiple open loop amplifiers connected in series).

It is generally desirable to provide a comparator with high bandwidth.Cascading an increasing number of amplifier stages yields a greaterbandwidth for the comparator. Cascading many amplifier stages toincrease bandwidth may, however, lower the gain at each amplifier stage.A decrease in the gain of each amplifier stage may undesirably result inthe comparator experiencing increased input-referred offset (i.e., thecomparator may suffer from inaccuracies when performing comparisonsbetween the first and second input signals). This necessitates the needfor offset-cancellation techniques, which requires additionalcompensation circuitry to be formed on the integrated circuit. Moreover,the use of multiple amplifier stages may consume a substantial amount ofpower during operation of the integrated circuit.

SUMMARY

Integrated circuits may include data converters such asanalog-to-digital converters. There are many different types ofanalog-to-digital (A/D) converters such as flash A/D converters,single-slope A/D converters, dual-slope A/D converters, successiveapproximation A/D converters, pipeline A/D converters, oversampled A/Dconverters, etc. A majority of these A/D converters may include at leasta latched comparator.

A latched comparator may have a first set of inputs configured toreceive a differential input signal, a second set of inputs configuredto receive a differential reference signal, a control input configuredto receive a clock (latching) signal, and an output. The latchedcomparator may be used to drive its output high if the differentialinput signal is greater in magnitude than the differential referencesignal or may be used to drive its output low if the differential inputsignal is lesser in magnitude than the differential reference signal.

The latched comparator may include at least a preamplifier, a latch, alevel shifter, and a flip-flop coupled in a chain. The latch andflip-flop may be controlled using the clock signal. The preamplifier mayinclude a first set of matching input transistors configured to receivethe differential input signal and a second set of matching inputtransistors configured to receive the difference reference signal. Thefirst and second set of matching input transistors may be connected at ashared tail node. At least two transistors may be coupled in seriesbetween the tail node and a ground power supply line for providingincreased small signal impedance at the tail node. The preamplifier mayalso include first and second sets of pull-up transistors connected toits output. The first and second sets of pull-up transistors may eachinclude a diode-connected transistor for minimizing kickback noise andmay include a cross-coupled pair for maximizing preamplifier gain.

The latch may include a first latch portion containing pull-uptransistors and a second latch portion containing pull-down transistors.Switches that are controlled using the clock signal may be turned on toelectrically connect the first and second latch portions and may beturned off to decouple the first and second latch portions. The firstlatch portion may include transistors that are coupled to the output ofthe preamplifier and may include a first latch cross-coupled pull-uptransistor pair.

The second latch portion may include precharge transistors configured toprecharge the latch output to a predetermined voltage level and mayinclude a second latch cross-coupled pull-down transistor pair. During afirst (high) clock phase of the clock signal, the first and secondportions may be decoupled from one another and the latch prechargetransistors may be turned on to precharge the latch output to thepredetermined voltage level (e.g., to a logic high, a logic low, or anintermediate voltage level). During a second (low) clock phase of theclock signal, the first and second portions may be electricallyconnected, and the first and second cross-coupled transistor pairs mayserve to provide exponential regeneration for the latch (e.g., to driveany voltage difference presented at the output of the preamplifier to alogic “1” or a logic “0”) while the precharge transistors are turnedoff.

Further features of the present invention, its nature and variousadvantages will be more apparent from the accompanying drawings and thefollowing detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative integrated circuit having atleast one analog-to-digital converter in accordance with an embodimentof the present invention.

FIG. 2 is a diagram of an illustrative digital-to-analog convertingcircuit that can be used in the analog-to-digital converter of FIG. 1 inaccordance with an embodiment of the present invention.

FIG. 3 is a table of illustrative encoding schemes that can be used bythe analog-to-digital converter of FIG. 1 in accordance with anembodiment of the present invention.

FIG. 4 is a high-level diagram of an illustrative latched comparatorcircuit in accordance with an embodiment of the present invention.

FIG. 5 is a plot of output voltage versus an input differential voltageillustrating the operation of a latched comparator circuit in accordancewith an embodiment of the present invention.

FIG. 6 is a timing diagram showing relevant waveforms during operationof a latched comparator circuit in accordance with an embodiment of thepresent invention.

FIG. 7 is a diagram of an illustrative latched comparator circuit havinga preamplifier stage and a latching stage in accordance with anembodiment of the present invention.

FIG. 8 is a circuit diagram of an illustrative preamplifier circuit inaccordance with an embodiment of the present invention.

FIG. 9 is a circuit diagram of an illustrative regenerative latchingcircuit in accordance with an embodiment of the present invention.

FIG. 10 is a timing diagram showing relevant waveforms during operationof the preamplifier circuit of FIG. 8 and the regenerative latchingcircuit of FIG. 9 in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION

Embodiments of the present invention relate to integrated circuits withdata converters. The integrated circuits may be digital signalprocessors, microprocessors, application specific integrated circuits(ASICs), programmable logic device integrated circuits (PLDs), or othertypes of integrated circuits.

FIG. 1 is a diagram of an integrated circuit 10 that includesinput-output (I/O) circuitry 12, storage and processing circuitry 14,and at least one analog-to-digital converter (ADC) 16. Input-outputcircuitry 12 may include driver circuits configured to drive signals offof device 10 via input-output pins, phase alignment circuits configuredto receive signals from other devices via the input-output pins,electrostatic discharge (ESD) protection circuitry configured to limitthe amount of electrostatic current that flows through the input-outputpins for preventing damage due to electrostatic discharge events,decoupling capacitor circuitry configured to serve as an energy reserveduring dynamic switching operations (e.g., to help provide more stablepower supply voltages), and other suitable input-output circuitry.

Storage and processing circuitry 14 may include one or more differenttypes of storage such as nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only memory), volatile memory (e.g.,static or dynamic random-access-memory), data registers, and/or othersuitable types of memory. Storage and processing circuitry 14 may alsoinclude logic circuits such as combinatorial logic circuits andsequential logic circuits, digital signal processing modules,radio-frequency processing modules (e.g., wireless communicationscircuitry such as baseband circuits, power amplifiers, low noiseamplifiers, filters, etc.), power management circuits, control circuitssuch as lookup tables, multiplexers, encoders/decoders, counters,adders, multipliers, and other arithmetic logic circuits, etc. Circuitry14 may also include control circuitry operable to control theinteraction among the different circuitry on device 10 and othersuitable digital and analog processing circuits.

As shown in FIG. 1, device 10 may include at least one analog-to-digital(A/D) converter 16 (sometimes abbreviated as ADC). Converter 16 may beconsidered to be part of storage and processing circuitry 14. Ingeneral, device 10 may include a plurality of analog-to-digitalconverters 16, a plurality of digital-to-analog converters (DACs), andassociated digital signal processing circuits (e.g., sample-and-holdcircuits, digital filters, analog filters, encoding/decoding circuits,etc.). Different types of integrated circuits may utilize different A/Dconverter architectures. Different analog-to-digital converterimplementations includes flash A/D converters, single-slope A/Dconverters, multi-slope A/D converters, successive approximation A/Dconverters, pipeline A/D converters, oversampled A/D converters, etc.

In the example shown in FIG. 1, device 10 may include a flash A/Dconverter 16. In general, flash A/D converters offer high speedoperation at relatively high resolutions (e.g., up to eight-bitprecision). Analog-to-digital converter 16 may include adigital-to-analog converter (DAC) 18 having a given number of outputs28, a corresponding number of comparator circuits 20, and an encodingcircuit (encoder) 22. Circuit 18 may be supplied with a reference powersupply voltage Vref over power supply line 90 and may have respectiveoutputs 28 at which varying levels of intermediate reference voltagesare provided.

Each comparator circuit 20 may include a first (positive) input at whichan input voltage Vin is received over path 24, a second (negative) inputconfigured to receive a selected one of the intermediate referencevoltages Vref<k> from circuit 18, a control input that receives clocksignal Clk over path 26, and an output. Comparator circuit 20 may beused to determine whether the voltage level at its first input isgreater than or less than the voltage level at its second input (e.g.,to determine whether Vin is greater or less than Vref<k>).

During a first (high) clock phase of control signal Clk, comparatorcircuit 20 may linearly amplify any voltage difference between Vin andselected reference voltage Vref<k>. During a second (low) clock phase ofClk, comparator circuit 20 may exponentially regenerate the amplifiedvoltage difference (e.g., circuit 20 will drive its output to logic “1”if Vin is greater than Vref<k> and will drive its output to logic “0” ifVin is less than Vref<k>). The result may be latched at its output atthe following clock edge (e.g., at the subsequent falling clock edge).Comparator circuit 20 controlled using clock signal Clk in this way maysometimes be referred to as a latched comparator.

Encoder circuit 22 may have inputs coupled to the outputs of comparators20 and may have an output on which digital signal Dout is generated. Thevalue of signal Dout may be proportional to the voltage magnitude ofVin. For example, consider a scenario in which A/D converter 16 is aneight-bit A/D converter (i.e., Dout is an eight-bit wide digitalsignal), is supplied with reference supply voltage Vref that is equal to1 V, and receives an input voltage Vin that is currently equal to 0.78V. In this example, DAC 18 may be formed as a resistive ladder thatincludes 2⁸ resistors (see, e.g., FIG. 2).

As shown in FIG. 2, DAC 18 may include 256 resistors coupled in seriesbetween reference power supply line 90 and ground power supply line 92(e.g., a ground power supply line on which ground power supply signalVss is provided). The topmost resistor 27 that is directly connected topower supply line 90 and the bottommost resistor 27 that is directlyconnected to power supply line 92 may have a resistance of R/2, whereaseach of resistors 27 that is coupled between the topmost resistor andthe bottommost resistor exhibits a resistance of R. The node at whicheach pair of adjacent resistors 27 connect may formed an intermediatetap point for DAC 18. The intermediate tap points may be couple torespective output paths 28 over which a predetermined pattern of voltagelevels is provided.

In this example, there may be 255 output paths 28 (2⁸ minus 1), where avoltage level Vref<0> at first output path 28 (i.e., an output path thattaps into the intermediate node between the bottommost resistor and asecond resistor that is adjacent to the bottommost resistor) is equal toequal to 1.96 mV (0.5/255*Vref), where a voltage level Vref<1> at secondoutput path 28 (i.e., an output path that taps into the intermediatenode between the second resistor and a third resistor that is adjacentto the second resistor) is equal to 5.88 mV (1.5/255*Vref), . . . ,where a voltage level Vref<254> of 255^(th) output path 28 (i.e., anoutput path that taps into the intermediate node between the topmostresistor and a resistor that is adjacent to the topmost resistor) isequal to 998 mV (254.5/255*Vref). The eight-bit A/D converter in thisexample therefore exhibits a least significant bit (LSB) resolution of3.92 mV (Vref/255). In other words, DAC 18 may be used to generate arange of voltage levels spaced apart at desired step sizes (e.g., thedifference between Vref<k+1> and Vref<k> is equal to the LSB spacing ofA/D converter 16). DAC 18 may also be formed using capacitive circuitsor other suitable resistive circuits, if desired.

As described previously, each comparator 20 may receive one ofintermediate reference voltages Vref<k> over a corresponding path 28 atits second input. Each comparator circuit 20 may receive input voltagesignal Vin at its first input. Consider an example in which theexemplary eight-bit A/D converter 16 described above receives a Vin thatis equal to 0.75 V. The 64 topmost output paths 28 will carryintermediate reference voltages that are greater than Vin (e.g.,Vref<254:191> are greater than 0.75 V), whereas the 191 remaining outputpaths 28 will carry intermediate reference voltages that are less thanVin (e.g., Vref<190:0> are less than 0.75 V). In this example, Vref<191>may be equal to 751 mV (191.5/255*1). As a result, the top 64comparators 20 may generate logic zeroes at their outputs while thebottom 191 comparators 20 may generate logic ones at their outputs. Anoutput pattern in which a group of logic “0s” (where the mostsignificant bits are zeroes) is followed by a group of logic “1s” (wherethe least significant bits are ones) may be referred to as a thermometercode representation (e.g., a base one numerical counting system).

Encoder 22 may be used to convert thermometer code based signals intoother numerical representations such as binary-code based signals,gray-code based signals, or other signals with other encoding schemes(see, e.g., FIG. 3). As shown in FIG. 3, encoder 22 may receive aneight-bit thermometer code based signal and may convert the eight-bitsignal into a three-bit gray code based signal Dout or a three-bitbinary code based signal Dout. In the 8-bit A/D converter exampledescribed in connection with FIG. 1, encoder 22 may receive a 255-bitthermometer code based signal at its input and may output an eight-bitbinary coded based signal at its output. In general, encoder 22 mayconvert the thermometer code based signals generated using comparators20 using any desired encoding scheme.

A/D converter 16 arranged using the exemplary configuration of FIG. 1offers high performance by requiring only one clock cycle perconversion. Latched comparator circuits 20 can serve as an importantbuilding block for many different types of data converters.

FIG. 4 shows a high level block diagram of a latched comparator 20. Asshown in FIG. 4, latched comparator 20 may have a first (positive) inputconfigured to receive a differential input voltage Vin_diff, a second(negative) input configured to receive a differential intermediatereference voltage Vref_diff<k>, a control input configured to receiveclock signal Clk, power supply terminals for receiving voltages Vcc andVss, and an output on which signal Vout is provided. The differencebetween voltages Vin_diff that is received at the first input andVref_diff<k> that is received at the second input may be referred to asa comparator differential input voltage Vid (e.g., Vid may be equal toVin_diff minus Vref_diff<k>).

FIG. 5 is a plot of output voltage Vout versus comparator differentialinput voltage Vid. As shown in FIG. 5, comparator 20 is configured todrive Vout low when Vid is less than a switching threshold Vsw and isconfigured to drive Vout high when Vid is greater than Vsw. Ideally,threshold Vsw should be equal to zero volts. In the presence of processvariations and manufacturing non-idealities, however, Vsw can sometimesshift to a positive value (as shown FIG. 5) or to a negative value dueto transistor mismatch. This undesirable shift is sometimes referred toas a comparator offset.

FIG. 6 is a timing diagram illustrating the operation of latchedcomparator 20. As shown in FIG. 5, comparator differential input voltageVid may be a time-varying analog voltage signal. Assuming that switchingthreshold Vsw is equal to zero volts, voltage Vid may sometimes begreater than zero volts or may sometimes be less than zero volts.

At time t1, signal Clk clocks high. When Clk is high, comparator 20 maybe placed in a linear amplification mode that amplifies any voltagedifference between Vin_diff and Vref_diff<k> (i.e., comparator 20 maypreamplify Vid during the high clock phase). In this example, Vid isgreater than zero volts during the high clock phase so the comparatorshould eventually latch a high data signal.

At time t2, signal Clk clocks low. When Clk is low, comparator 20 may beplaced in an exponential amplification mode that drives an internaloutput node in comparator 20 to logic “1” if the preamplified voltagesignal is positive or to logic “0” if the preamplified voltage signal isnegative. The exponential amplification mode may sometimes be referredto as an exponential regeneration phase (e.g., comparator 20 may includegain circuits connected in a positive feedback configuration, which canhelp drive signals to logic high/low). At time t3 (when Clk risesagain), the regenerated data may be latched at the output of comparator20 (e.g., Vout is latched high). Prior to latching valid data, Vout mayhave an unknown voltage (see, e.g., Vout prior to time t3).

For the remainder of the high clock phase immediately following time t3,a subsequent data comparison may be performed. As shown in this example,only one clock cycle is required to obtain new valid data (e.g., a halfclock cycle for preamplification and a half clock cycle forregeneration). In this example, new data may be latched at each risingedge of signal Clk. Comparator 20 may be used during normal operation ofA/D converter 16 to perform data comparisons in real time.

Latched comparator 20 may include several different circuits coupled ina chain (see, e.g., FIG. 7). As shown in FIG. 7, comparator 20 mayinclude at least a preamplifier 70, a latching circuit 72, a levelshifting circuit 74, and a flip-flop 76. Preamplifier 70 may have afirst (positive) input that serves as the first input of comparator 20(e.g., for receiving Vin_diff), a second (negative) input that serves asthe second input of comparator 20 (e.g., for receiving Vref_diff<k>),and a differential output. The differential output of preamplifier 70may be coupled to inputs of latch 72.

Preamplifier 70 may serve to perform linear amplification on Vid,whereas latch 72 may serve to perform exponential regeneration on theamplified versioned of Vid. The regenerated signal may be fed throughlevel shifter 74 and may then be latched using flip-flop 76 (e.g., anegative-edge-triggered flip-flop). Signal Vout may be presented at theoutput of flip-flop 76. Preamplifier 70 may be designed to exhibitreduced offset by forming relatively large input transistors, because itis generally easier to fabricate large matching transistors as opposedto minimum sized matching transistors. Moreover, preamplifier 70 mayserve to further reduce the input-referred offset associated with latch72 by the gain of preamplifier 70.

The use of preamplifier 70 prior to latching circuit 72 may also serveto reduce kickback noise. Kickback noise may be caused by parasiticcapacitive coupling generated when latching signal Clk rises high orfalls low. Because preamplifier 70 does not include any transistor thatis controlled using Clk, preamplifier 70 effectively isolates the inputof comparator 20 from any undesired Clk activity that is present inlatching circuit 72, thereby minimizing kickback.

FIG. 8 is a circuit diagram of preamplifier 70. Preamplifier 70 may havea first set of inputs (described collectively as the positive input ofpreamplifier 70 in FIG. 7) at which Vi_pos and Vi_neg are received and asecond set of inputs (described collectively as the negative input ofpreamplifier 70 in FIG. 7) at which Vr_pos and Vr_neg are received. Thedifference between Vi_pos and Vi_neg represents Vin_diff, whereas thedifference between Vr_pos and Vr_neg represents Vref_diff<k>.Preamplifier 70 may also have first output node 116 and second outputnode 118 at which output voltages Vo_pos and Vo_neg are provided,respectively. Preamplifier output nodes 116 and 118 may serve as thedifferential output of preamplifier 70.

Preamplifier 70 may include n-channel devices (e.g., n-channelmetal-oxide-semiconductor transistors) such as n-channel transistors100, 102, 104, 106, 130, and 132 and p-channel devices (e.g., p-channelmetal-oxide-semiconductor transistors) such as p-channel transistors108, 110, 112, and 114. P-channel transistors 108 and 112 may each havesource-drain terminals that are coupled between power supply line 94 andpreamplifier output node 118 (e.g., transistors 108 and 112 may becoupled in parallel between line 94 and node 118). Transistors 110 and114 may each have source-drain terminals that are coupled between powersupply line 94 and preamplifier output node 116 (e.g., transistors 110and 114 may be coupled in parallel between line 94 and node 116).Transistor 108 may have a gate that is coupled to output node 116,whereas transistor 110 may have a gate that is coupled to output node118. Cross-coupling p-channel transistors 108 and 110 in this waymaximizes the gain of preamplifier 70.

Transistor 112 may have a gate that is coupled to output node 118, andtransistor 114 may have a gate that is coupled to output node 116.Transistors 112 and 114 in which the gate and drain terminals areshorted may sometimes be referred to as “diode-connected” transistors.Forming output load transistors 112 and 114 in the diode-connectedconfiguration may reduce the high frequency (small signal) impedance atoutput nodes 116 and 118, which can also help reduce kickback noise fromlatch 72.

N-channel transistor 100 may have a drain terminal that is coupled tooutput node 118, a source terminal that is coupled to tail node 120, anda gate that is controlled by Vi_pos. Transistor 102 may have a drainterminal that is coupled to output node 116, a source terminal that iscoupled to tail node 120, and a gate that is controlled by Vi_neg.Transistor 104 may have a drain terminal that is coupled to output node116, a source terminal that is coupled to tail node 120, and a gate thatis controlled by Vr_pos. Transistor 106 may have a drain terminal thatis coupled to output node 118, a source terminal that is coupled to tailnode 120, and a gate that is controlled by Vr_neg. Transistors 100, 102,104, and 106 may serve as input transistors for preamplifier 70 and maybe relatively large transistors (e.g., compared to p-channel transistors108 and 112) such that preamplifier offset is minimized.

N-channel transistors 130 and 132 may be coupled in series between tailnode 120 and ground line 92. Transistors 130 may have a gate thatreceives first bias voltage Vb1, whereas transistors 132 may have a gatethat receives second bias voltage Vb2. Voltages Vb1 and Vb2 may beconfigured such that the current flowing through transistors 130 and 132can provide sufficient gain for preamplifier 70. Stacking (or“cascoding”) transistors 130 and 132 in this way can also help increasethe high frequency impedance at tail node 12, further stabilizing theoperation of preamplifier 70.

FIG. 9 is a circuit diagram of latch (sometimes referred to as theregenerative stage) 72. Latch 72 may have a first input configured toreceive signal Vo_pos from output node 116 of preamplifier 70, a secondinput configured to receive signal Vo_neg from output node 118 ofpreamplifier 70, and a control input configured to receive signal Clk.Latch 72 may also have first output node 234 and second output node 236at which latch output voltages Lo_pos and Lo_neg are provided,respectively. Latch output nodes 234 and 236 may serve as a differentialoutput for latch 72.

Latch 72 may be operable in first and second modes. When signal Clk ishigh, circuit 72 may be placed in a first (idle) mode in which latch 72is configured to precharge its output nodes to a predetermined voltagelevel (as an example). During this time, preamplifier 70 performs linearamplification on Vid and presents the preamplified version of Vid at thefirst and second inputs of latch 72. When signal Clk is low, circuit 72may be placed in a second (active) mode in which latch 72 is configuredto exponentially regenerate any voltage difference between Vo_pos andVo_neg. For example, if Vo_pos is greater than Vo_neg, latch 72 maydrive Lo_pos to Vcc and Lo_neg to ground. As another example, if Vo_posis less than Vo_neg, latch 72 may drive Lo_pos to ground and Lo_neg toVcc.

Latching circuit 72 may include a first latch portion 200 and a secondlatch portion 202. Transistors controlled by signal Clk may bridge thefirst and second portions. During the first idle latch mode, the firstand second portions may be decoupled from one another (e.g., so thatselected precharge transistors are turned on). During the second mode,the first and second portions may be electrically connected to providedesired exponential regeneration (while the precharge transistors areturned off).

First portion 200 of latch 72 may include p-channel transistors 204,206, 208, and 210, whereas second portion 202 of latching circuit 72 mayinclude n-channel transistors 216, 218, 220, 222, 224, 226, and 228.Transistors 212 and 214 may serve to bridge the first and secondportions. Transistor 212 and 214 may be p-channel transistors (as anexample). Transistors 204 and 208 may have source terminals that arecoupled to power supply line 94 and drain terminals that are coupled tofirst intermediate node 230, whereas transistors 206 and 210 may havesource terminals that are coupled to power supply line 94 and drainterminals that are coupled to second intermediate node 232. Transistor204 may have a gate that receives signal Vo_pos, whereas transistor 206may have a gate that receives signal Vo_neg. Transistor 208 may have agate that is coupled to node 232, and transistor 210 may have a gatethat is coupled to node 230. Cross-coupling p-channel transistors 208and 210 in this way may help provide high gain for latch 72.

P-channel transistor 212 may be coupled between node 230 and latchoutput node 236, whereas p-channel transistor 214 may be coupled betweennode 232 and latch output node 234. Transistors 212 and 214 may havegates that are controlled using signal Clk. When Clk is low, firstportion 200 and second portion 202 of latch 72 may be connected toprovide exponential regeneration.

Transistor 216 may be coupled between latch output node 236 and thirdintermediate node 238, and transistor 218 may be coupled between latchoutput node 234 and fourth intermediate node 240. Transistors 216 and218 may have gates that are controlled by enable signal EN. Signal ENmay be low during reset operations and may generally be held high duringnormal operation of comparator 20. Transistors 216 and 218 need not beused.

Transistor 220 may have a first source-drain terminal that is coupled tonode 238, a second source-drain terminal that is coupled to node 240,and a gate that is controlled by signal Clk. Transistor 220 may serve toequalize the voltage level between nodes 238 and 240 when signal Clk ishigh.

Transistor 222 and 226 may have drain terminals that are coupled to node238 and source terminals that are coupled to ground line 92. Transistors224 and 228 may have drain terminals that are coupled to node 240 andsource terminals that are coupled to ground line 92. Transistors 222 and224 may have gates that are controlled by signal Clk and may serve toprecharge latch output voltages Lo_pos and Lo_neg (with the help oftransistor 220) to zero volts when Clk is high, as an example.Transistors 222, 224, and 220 may therefore be referred to as latchprecharge transistors. Transistor 226 may have a gate that is coupled tonode 240, whereas transistor 228 may have a gate that is coupled to node238. N-channel transistors 226 and 228 cross-coupled using thisarrangement may help provide high gain for latch 72. Cross-coupledpull-up transistors 208 and 210 and cross-coupled pull-down transistors226 and 228 collectively form two inverting circuits that are coupled ina positive feedback configuration. A pair of cross-coupled invertingcircuits may serve to provide exponential regeneration for latch 72.

FIG. 10 is a timing diagram that further illustrates the operation oflatched comparator 20. As shown in FIG. 10, Vin_diff (i.e., Vi_pos minusVi_neg) may vary in time while Vref_diff<k> stays fixed at a constantvalue (for a particular comparator 20). Preamplifier 70 may be used toamplify the difference between Vin_diff and Vref_diff<k> so preamplifieroutput differential voltage (i.e., Vo_pos minus Vo_neg) swings between 2V and 3 V (assuming Vcc is equal to 3 V in this example).

At time t1, signal Clk may rise high, triggering latch to precharge bothsignals Lo_pos and Lo_neg to zero volts. From time t1 to t2,preamplifier 70 may be used to generate amplified differential voltage(Vo_pos minus Vo_neg) at the input of latch 72. At time t2, signal Clkfalls low and latch 72 is placed in active mode. When placed in theactive mode, latch 72 regenerates the amplified differential voltage byproviding exponential amplification, which drives the latch outputsignals either high or low (e.g., latch 72 will amplify any voltagedifference at its input so that its output will be driven to logic “1”or logic “0”). In the example of FIG. 10, Lo_pos is driven to Vcc andLo_neg is driven to Vss because the difference between Vin_diff andVref_diff<k> is positive at least during the sensing time period betweent1 and t2. At time t3, Clk rises again to precharge the latch outputvoltages in preparation for sensing a subsequent data point.

In this example, level shift circuit 74 may receive latch output voltagesignals Lo_pos and Lo_neg from latch 72 and downshift the signals from 3V to 1 V (as an example). If desired, level shifter 74 may perform anydesired voltage up-conversion or down-conversion. The level-convertedvoltage may then be fed to flip-flop 76 and may be latched at eachrising clock edge of Clk (e.g., at time t3).

Details of latched comparator 20 described in connection with FIGS. 6-10are merely illustrative and do not serve to limit the scope of thepresent invention. If desired, other types of preamplification circuitmay be used, any number of preamplifying stages may be used, other typesof latching stages having more than less than two pairs of cross-coupledtransistors may be used, other types of clocking schemes may be used,etc.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention. Theforegoing embodiments may be implemented individually or in anycombination.

The invention claimed is:
 1. A comparator circuit comprising: anamplifier having first and second input transistors operable to receivefirst differential input signals and having third and fourth inputtransistors operable to receive second differential input signals,wherein the first input transistor is coupled between a first amplifieroutput and a tail node, wherein the second input transistor is coupledbetween a second amplifier output and the tail node, wherein the thirdinput transistor is coupled between the second amplifier output and thetail node, and wherein the fourth input transistor is coupled betweenthe first amplifier output and the tail node; at least one tailtransistor having a first source-drain terminal that is coupled to apower supply line and a second source-drain terminal that is directlycoupled to the first, second, third, and fourth input transistors at thetail node; and a latching circuit having inputs that are coupled to thefirst and second amplifier outputs.
 2. The comparator circuit defined inclaim 1 wherein the amplifier further comprises: a fifth transistor thatis coupled between a power supply line and the first amplifier output;and a sixth transistor that is coupled between the power supply line andthe second amplifier output, wherein the fifth transistor has a gatethat is coupled to the second amplifier output and wherein the sixthtransistor has a gate that is coupled to the first amplifier output. 3.The comparator circuit defined in claim 2 wherein the amplifier furthercomprises: a seventh transistor that is coupled between the power supplyline and the first amplifier output; and an eighth transistor that iscoupled between the power supply line and the second amplifier output,wherein the seventh transistor has a gate that is coupled to the firstamplifier output and wherein the eighth transistor has a gate that iscoupled to the second amplifier output.
 4. The comparator circuitdefined in claim 1 wherein the amplifier further comprises: a fifthtransistor that is coupled between a power supply line and the firstamplifier output; and a sixth transistor that is coupled between thepower supply line and the second amplifier output, wherein the fifthtransistor has a gate that is coupled to the first amplifier output andwherein the sixth transistor has a gate that is coupled to the secondamplifier output.
 5. A comparator circuit comprising: an amplifier thatincludes first and second input transistors operable to receive firstdifferential input signals and that includes at least third and fourthtransistors, wherein the first input transistor is coupled between afirst amplifier output and a tail node, wherein the second inputtransistor is coupled between a second amplifier output and the tailnode, and wherein the at least third and fourth transistors are coupledtogether in series between the tail node and a first power supply line;and a latching circuit having inputs that are coupled to the first andsecond amplifier outputs.
 6. The comparator circuit defined in claim 5wherein the amplifier further comprises: fifth and sixth inputtransistors operable to receive second differential input signals thatare different than the first differential input signals, wherein thefifth input transistor is coupled between the second amplifier outputand the tail node and wherein the sixth input transistor is coupledbetween the first amplifier output and the tail node.
 7. The comparatorcircuit defined in claim 5 wherein the amplifier further comprises: afifth transistor coupled between a second power supply line and thefirst amplifier output; and a sixth transistor coupled between thesecond power supply line and the second amplifier output, wherein thefirst and second power supply lines carry different power supply voltagelevels, wherein the fifth transistor has a gate that is coupled to thesecond amplifier output, and wherein the sixth transistor has a gatethat is coupled to the first amplifier output.
 8. The comparator circuitdefined in claim 7 wherein the amplifier further comprises: a seventhtransistor that is coupled in parallel with the fifth transistor; and aneighth transistor that is coupled in parallel with the sixth transistor,wherein the seventh transistor has a gate that is coupled to the firstamplifier output and wherein the eighth transistor has a gate that iscoupled to the second amplifier output.
 9. The comparator circuitdefined in claim 5 wherein the latching circuit comprises: a first setof transistors having gates operable to serve as the inputs of thelatching circuit; a second set of transistors having gates operable toreceive a clock signal; and a third set of transistors interposedbetween the first and second sets of transistors, wherein the third setof transistors have gates operable to receive the clock signal.
 10. Thecomparator circuit defined in claim 9 wherein the third set oftransistors comprises p-channel transistors.
 11. A comparator circuitcomprising: an amplifier having a first input operable to receive afirst differential input signal, a second input operable to receive asecond differential input signal, and an output on which an amplifierdifferential output signal is generated, wherein the first and secondinputs simultaneously receive the first and second differential inputsignals; and a latching circuit having a first input operable to receivethe amplifier differential output signal from the amplifier and a secondinput operable to receive a clock signal.
 12. A comparator circuitcomprising: an amplifier having a first input operable to receive afirst differential input signal, a second input operable to receive asecond differential input signal, and an output on which an amplifierdifferential output signal is generated; and a latching circuit having afirst input operable to receive the amplifier differential output signalfrom the amplifier and a second input operable to receive a clocksignal, wherein the latching circuit includes a first latch portion anda second latch portion, wherein the first and second latch portions arecoupled to each other when the clock signal is deasserted, and whereinthe first and second latch portions are decoupled from each other whenthe clock signal is asserted.
 13. The comparator circuit defined inclaim 12, wherein the first latch portion comprises a first pair ofcross-coupled transistors and wherein the second latch portion comprisesa second pair of cross-coupled transistors.
 14. The comparator circuitdefined in claim 12, further comprising: a pair of transistors coupledto the second latch portion, wherein the pair of transistors have gatesoperable to receive the clock signal.
 15. A comparator circuitcomprising: an amplifier having a first input operable to receive afirst differential input signal, a second input operable tosimultaneously receive a second differential input signal, and an outputon which an amplifier differential output signal is generated; alatching circuit having a first input operable to receive the amplifierdifferential output signal from the amplifier and a second inputoperable to receive a clock signal; a level shifting circuit coupled toan output of the latching circuit; and a flip-flop coupled to an outputof the level shifting circuit, wherein the flip-flop has a control inputoperable to receive the clock signal.